Combustion chamber for a supercharged direct-injection combustion engine

ABSTRACT

A piston for an internal combustion engine, for example a diesel engine, including a body delimited laterally by a skirt and configured to collaborate with walls of a cylinder of axis of revolution C in which the piston can slide along the axis C, the piston including a transverse face that includes a central pip, a peripheral ring, and a bowl of axis of revolution B that extends from the central pip towards the peripheral ring to which it connects at a lip of thickness Ep. The bowl includes, in substantial vertical alignment with the lip, a torus in profile, for example of a dome-shaped, of maximum radius Rt capable of guiding fuel injected under the lip in a region of a re-entrant zone R towards the central pip. The tip of the pip lying on the axis of revolution B of the bowl rises to a height that is a distance Dt of between 5.4 mm and 8 mm, or substantially equal to 7.2 mm, below the level of the peripheral ring.

The invention relates in general to the design of combustion engines, particularly compression-ignition internal combustion engines.

Emissions standards imposed on motor vehicle manufacturers are becoming increasingly severe, each change in a standard resulting in significant technical developments and in the use of additional and/or more complicated emissions-control devices which prove to be expensive.

Future standards are encouraging manufacturers especially to reduce the level of exhaust emissions of nitrogen oxides and particulates, notably in order not to clog the particulate filters too quickly, but the manufacturers are at the same time wishing to increase, or at least to maintain, the engine performance levels and drivability.

Solutions generally adopted to increase emissions control of engines for example consist in resorting to advanced post-treatment devices such as particulate filters or in modifying the exhaust gas recirculation specifications, the English acronym EGR being widely recognized in this field, although this has an adverse impact on engine efficiency.

It is an object of the present invention to propose a combustion chamber that is improved and that is able notably to reduce the level of nitrogen oxides and particulates exhaust emissions.

One subject of the invention is a piston for an internal combustion engine, notably for a diesel engine, comprising a body laterally delimited by a skirt able to collaborate with the walls of a cylinder of axis of revolution C in which the piston is able to slide along this axis C, said piston comprising a transverse face which comprises a central pip, a peripheral crown ring and a bowl of axis of revolution B which extends from the central pip toward the peripheral crown ring to which it is connected at a lip of thickness Ep, said bowl comprising, substantially in vertical alignment with the lip, a torus in profile, preferably half-dome shaped, of maximum radius Rt, able to guide a fuel injected under the lip in a reentrance zone R toward the central pip, notable in that the top of the pip situated on the axis of revolution B of the bowl rises up to a height that is a distance Dt of between 5.4 mm and 8 mm, and preferably substantially equal to 7.2 mm below the height of the peripheral crown ring.

Use of a piston according to the invention notably allows the following advantages to be realized:

-   -   the post-treatment systems do not have to be made more         complicated and more expensive;     -   the nitrogen oxides emissions are reduced in order to comply         with emissions control standards.

According to some particular embodiments, the piston has one or more of the following features:

-   -   the central pip slopes toward the torus by an angle A, measured         from the axis of revolution B of the bowl in the geometric         direction, of between 65° and 65.7° and preferably substantially         equal to 65°;     -   the bowl is centered in the cylinder, the axis of revolution B         of the bowl coinciding with the axis of revolution C of the         cylinder;     -   the thickness Ep of the lip is between 4 mm and 6 mm and         preferably substantially equal to 5.3 mm;     -   the maximum radius of curvature Rt of the torus is between 5 mm         and 6 mm and preferably substantially equal to 5.2 mm;     -   the lip is situated a distance De/2 from the axis of revolution         B of the bowl, the distance De being between 49.6 mm and 51.5 mm         and preferably substantially equal to 50 mm;     -   the torus is situated a distance Db/2 from the axis of         revolution B of the bowl, the distance Db being between 53.8 mm         and 55.4 mm and preferably substantially equal to 54 mm;     -   the difference in distances, with respect to the axis of         revolution B of the bowl, of the end of the crown ring De/2 and         of the end of the torus Db/2 is substantially equal to 2 mm;     -   the maximum depth P of the bowl is between 14.8 mm and 16.2 mm         and preferably substantially equal to 16.2 mm.

A further subject of the invention is an internal combustion engine designed for strict emissions control standards in terms of the emissions of nitrogen oxides and particulates and, more specifically, an engine of the diesel type comprising at least one piston according to the invention.

This internal combustion engine of the diesel type has a cylinder of axis of revolution C the upper end of which is closed by a cylinder head provided with an underside which contributes to defining a combustion chamber together with the transverse face of the piston the piston bowl of which is centered about an axis of revolution B that coincides with the axis C, said engine comprising, opening onto the underside of the cylinder head, at least one inlet duct that can be closed off by an inlet valve and at least one exhaust duct that can be closed off by an exhaust valve, a preheater plug and a fuel injector the tip of which opens into the combustion chamber substantially at the axis of revolution of the cylinder C.

Other features and advantages of the invention will become clearly apparent from the description of the embodiment thereof given hereinafter by way of entirely nonlimiting indication with reference to the following figures:

FIG. 1 is a partial schematic cross section of an internal combustion engine according to the invention;

FIG. 2 is a partial cross section on a plane containing the axis of revolution of the piston bowl, showing the detail of the crown part of a piston according to the invention, on the inlet side.

FIG. 1 illustrates an internal combustion engine 100, notably for a diesel engine, comprising at least one cylinder 1 of axis of revolution C, a cylinder head 2 and a piston 3.

In the remainder of the description, this axis of revolution C will be considered to be oriented upward, toward the cylinder head 2.

The piston 3 is mounted in the cylinder 1 such that it can slide along the axis of revolution of the cylinder C, and has a piston body designed to accept a pin to connect the piston to the little end of a driving connecting rod, said body being laterally delimited by a piston skirt 31 running parallel to the axis of revolution of the cylinder C and collaborating with an internal wall of the cylinder 1. The piston further comprises a transverse face 32 which, together with the underside 20 of the cylinder head 2, contributes to delimiting a combustion chamber of the cylinder 1.

The fresh air or a mixture of fresh air and of recirculated exhaust gases is admitted to the combustion chamber via at least one inlet duct 5 formed in the cylinder head 2 and which can be closed off by at least one inlet valve 50.

The residue of combustion of the air-fuel mixture introduced is removed via at least one exhaust duct 6 formed in the cylinder head 2 and which can be closed off via at least one exhaust valve 60.

A preheater plug 4 is fitted in the cylinder head 2, its end opening into the combustion chamber so that it can heat the air-fuel mixture during cold starts.

A fuel injector 7 is fitted in the cylinder head 2 and opens into the combustion chamber substantially along the axis of revolution C of the cylinder 1.

As depicted in FIG. 2 which, in partial cross section on an axial plane, details the upper part of a piston 3 according to the invention, the transverse face 32 comprises a central pip 321, a peripheral crown ring 322 and an annular cavity or bowl 323 of axis of revolution B that coincides with the axis of revolution of the cylinder C, said bowl 323 extending from the central pip 321 toward the peripheral crown ring 322 to which it is connected.

As an alternative, the bowl 323 may be offset in the cylinder 1, the axes of revolution C of the cylinder 1 and B of the bowl 323 therefore being able to be offset from one another by a distance preferably shorter than a value substantially equal to 3 mm.

The peripheral crown ring 322 extends laterally from the skirt 31 of the piston 3 toward the axis of revolution B of the bowl 323 as far as an end that forms a lip 3220 underneath which the bowl 323 has a torus 3230 in profile half-dome shaped, with a maximum radius of curvature Rt of between 5 mm and 7 mm and preferably substantially equal to 6 mm.

The lip 3220 is situated a distance De/2 from the axis of revolution B of the bowl 323, the distance De being between 49.6 mm and 51.5 mm and preferably substantially equal to 50 mm.

The torus 3230 is situated a distance Db/2 from the axis of revolution B of the bowl 323, the distance Db being between 53.8 mm and 55.4 mm and preferably substantially equal to 54 mm.

The injector 7 is designed selectively to inject fuel in the form of jets directed into an upper region of the torus 3220 adjacent to a bottom edge of the lip 3220, also known as a reentrant edge R, so as to improve the way in which the jet of fuel is guided from this reentrant edge R by rolling along the walls of the torus 3230 toward the bottom of the bowl, where the oxygen is to be found as the piston 3 effects its upstroke, so as to reduce smoke and so as to enable the gases to be circulated toward the central pip 321.

For preference, a reentrant edge R will be maintained by keeping the difference in distances, with respect to the axis of revolution B of the bowl 323, of the end of the crown ring De/2 and of the end of the torus Db/2 substantially equal to 2 mm.

For preference, the thickness Ep of the lip 3220 corresponding to the distance of the reentrant edge R of the peripheral crown ring 322 is between 4 mm and 6 mm and preferably substantially equal to 5.3 mm.

The maximum depth P of the bowl 323 is between 14.8 mm and 16.2 mm and preferably substantially equal to 16.2 mm. The ratio Db/P gives the bowl 323 a substantial width which allows effective exploitation of the air under full load conditions, making it possible to achieve high specific performance, even with low swirl rates thus limiting the risk of the jets emitted by the injector 7 overlapping.

Combining this design of the reentrant edge R, with the torus 3230 and with a maximum bowl 323 depth P allows improved guidance of the jet of fuel toward a volume of air trapped in the bottom of the bowl 323.

The top of the central pip 321, situated on the axis of revolution B of the bowl, rises up to a height that is a distance Dt below the height of the peripheral crown ring, this distance Dt being between 5.4 mm and 8 mm, and preferably substantially equal to 7.2 mm. The distance Dt to the top of the peripheral crown ring makes it possible to limit interactions between the jets of fuel sprayed toward the bottom of the bowl with the central pip 321 and notably the top thereof, thus making it possible to reduce emissions of smoke and unburnt fuel.

The central pip 321 slopes by an angle A, measured from the axis of revolution B of the bowl in the geometric direction, of between 65° and 65.7° and preferably substantially equal to 65°.

Having the central pip 321 slope at an angle A like this allows the soot formed by combustion at the bottom of the bowl 323 to be extracted toward the central pip 321 for improved post-oxidation leading to a reduction in smoke.

An internal combustion engine 100 provided with a piston 3 combining these features proves to be particularly advantageous in use by comparison with an internal combustion engine provided with a piston suited to current emissions control standards, of the Euro 4 type. These tests were conducted using an engine 100 that had the following additional features: a bore of 85 mm, a compression ratio of about 15 and a level of swirl at bottom dead center Nd/N of about 3.

What was actually found was that under part-load conditions and for the same level of particulate emissions, this engine 100 emitted approximately 15% less nitrogen oxides.

For the same power and the same temperature before the turbine TAVT, at 4000 rpm and at full load, the amount of smoke was lower, and the smoke index was reduced by a value substantially equal to 0.5 fsn. 

1-10. (canceled)
 11. A piston for an internal combustion engine, comprising: a body laterally delimited by a skirt configured to collaborate with walls of a cylinder of axis of revolution C in which the piston can slide along the axis C, the piston comprising a transverse face that comprises a central pip, a peripheral crown ring, and a bowl of axis of revolution B that extends from the central pip toward the peripheral crown ring to which it is connected at a lip of thickness Ep, the bowl comprising, substantially in vertical alignment with the lip, a torus in profile, or of half-dome shaped, of maximum radius Rt, configured to guide a fuel injected under the lip in a re-entrance zone R toward the central pip, wherein the top of the pip situated on the axis of revolution B of the bowl rises up to a height that is a distance Dt of between 5.4 mm and 8 mm, or is substantially equal to 7.2 mm, below the height of the peripheral crown ring.
 12. The piston as claimed in claim 11, wherein the central pip slopes toward the torus by an angle A, measured from the axis of revolution B of the bowl in the geometric direction, of between 65° and 65.7°, or substantially equal to 65°.
 13. The piston as claimed in claim 13, wherein the bowl is centered in the cylinder, the axis of revolution B of the bowl coinciding with the axis of revolution C of the cylinder.
 14. The piston as claimed in claim 11, wherein the thickness Ep of the lip is between 4 mm and 6 mm or is substantially equal to 5.3 mm.
 15. The piston as claimed in claim 11, wherein the maximum radius of curvature Rt of the torus is between 5 mm and 7 mm, or is substantially equal to 6 mm.
 16. The piston as claimed in claim 11, wherein the lip is situated a distance De/2 from the axis of revolution B of the bowl, the distance De being between 49.6 mm and 51.5 mm or substantially equal to 50 mm.
 17. The piston as claimed in claim 11, wherein the torus is situated a distance Db/2 from the axis of revolution B of the bowl, the distance Db being between 53.8 mm and 55.4 mm or substantially equal to 54 mm.
 18. The piston as claimed in claim 11, wherein the difference in distances, with respect to the axis of revolution B of the bowl, of the end of the crown ring De/2 and of the end of the torus Db/2 is substantially equal to 2 mm.
 19. The piston as claimed in claim 11, wherein the maximum depth P of the bowl is between 14.8 mm and 16.2 mm, or is substantially equal to 16.2 mm.
 20. An internal combustion engine of diesel type, comprising at least one piston as claimed in claim
 13. 